This invention relates generally to single phase electric motor starters and more particularly to a universal motor starter for such motors.
The utilization of solid state switches for motor starting to improve reliability and longevity over conventional electromechanical relays is well known. Typically a gate controlled solid state switch, such as a triac, is serially connected to the start winding of a motor and is adapted upon initial energization of the motor to be gated into a low impedance state thereby permitting current flow in the start winding. After a brief period of time the gating current to the triac is interrupted causing the triac to go into a high impedance state to effectively deenergize the start winding. Many different approaches have been made, with varying degrees of success, to utilize one or more characteristics of the motor to prevent conduction of the triac and hence effect deenergization of the start winding at the optimum moment. For example, as disclosed in General Electric Application Note 200.35-3/66 page 16, line current is used to turn on the triac which drops out once the current settles down to normal levels. In U.S. Pat. No. 3,414,789 to Prouty main winding current is used to control the conductive state of the triac by means of the voltage across an impedance serially connected to the main winding. In U.S. Pat. No. 3,671,830 to Kruper the voltage across the start winding is used to control conduction of the triac through a Schmitt trigger arrangement. In U.S. Pat. No. 3,421,064 to Phillips a control winding, magnetically coupled to the main winding, develops a voltage vector which is compared with a voltage vector developed across a portion of the main winding with the vector difference used to control the conduction of the triac. In U.S. Pat. No. 3,746,951 to Hohman impedance elements are connected across the main winding to monitor motor speed by sensing the relative phase difference between start winding current and applied voltage to control the conductive state of the triac. U.S. Pat. No. 3,777,232 to Woods et al also employs phase angle relationships to control conduction of the triac by comparing the phase difference between main winding current and applied voltage in one embodiment and between main winding current and start winding current in another embodiment. In U.S. Pat. No. 4,307,327 to Streater et al the phase angle between start winding current and line current is used to trigger the triac through a reed switch disposed in the trigger circuit of the triac.
All of the above approaches suffer from one or more limitations with regard to their usefulness. For example, in several of the above including the General Electric approach, Prouty and Kruper, variations in voltage supply and loading effect the motor speed at which the start winding is deenergized resulting in inconsistent performance. Another disadvantage common to several of the circuits is that they require specific tailoring for them to be effective for a given motor. This is true of Phillips, Streater et al and Hohman. The approach of Woods et al suffers from a reliability problem since a triac is located in the main winding circuit and is adapted to be energized concomittantly with the main winding. The Woods et al circuit is also relatively complex and is inherently expensive due to the many components employed therein.
In addition to the above noted disadvantages, many of the above noted prior art circuits permit reenergization of the start winding under certain conditions to provide extra torque however in many applications, this can have an adverse effect on reliability and longevity which, as mentioned supra, are two of the main reasons for using triacs. For example, in cases where the motor might be subjected to continuous restarting, particularly under a heavy load such as a stall condition, there is a danger that the junction temperature of the triac could rise too high and the triac burn out.